The expression of cell surface receptors for viruses and hormones is being studied using animal models of human diseases. The induction and modulation of receptors for encephalomyocarditis virus and insulin have been studied, and the results have shown that virus and hormone receptors can be regulated in response to a variety of in vivo and in vitro stimuli. Evidence that receptors can determine tissue tropisms in vivo has been found using variants of encephalomyocarditis and mengovirus. Results of insulin binding studies have shown that under certain conditions, insulin receptor activity of leukocytes may be altered while receptor activities of other tissues remain normal. Thus, for some patients, especially those suffering from immunologically-mediated diseases, results of insulin binding assays using leukocytes may not truly reflect insulin receptor activity on other cell types.